Evidence for the cation-pi interaction between Cu2+ and tryptophan.
The cation-pi interaction, a noncovalent interaction of electrostatic nature between a cation and an electron-rich pi system, is increasingly recognized as an important force that influences the structures and functions of molecules including proteins. Unlike other metal cations, the transition metal cation Cu2+ is not regarded to take part in a cation-pi interaction because Cu2+ tends to oxidize the pi electron system, in particular that of Trp, and to introduce covalency in the metal-pi electron interaction. This paper reports the first spectral evidence for the cation-pi interaction between Cu2+ and Trp. The Cu2+ ion bound to the amino N-terminal Cu2+/Ni2+ binding motif composed of three amino acid residues interacts with the indole ring of the fourth Trp residue in a noncovalent manner. The Cu2+-Trp interaction produces a distinct negative band at 223 nm in circular dichroism (CD), which disappears upon mutation or depletion of the Trp residue or upon replacement of the Cu2+ ion by Ni2+. In UV absorption, a pair of negative/positive intensity changes is generated at 222/231 nm by the Cu2+-Trp interaction, being consistent with the previous observations on the indole ring interacting with K+ or a cationic His imidazole ring. The negative CD band around 223 nm is characteristic of the Cu2+-Trp pair and may be useful as a marker of the Cu2+-Trp cation-pi interaction. Coordination of negatively charged ligands to Cu2+ is suggested to be important for the cation to be involved in a cation-pi interaction.
Yorita
Hanami
H
Graduate School of Pharmaceutical Sciences, Tohoku University, Aobayama, Sendai 980-8578, Japan.
Otomo
Kohei
K
Hiramatsu
Hirotsugu
H
Toyama
Akira
A
Miura
Takashi
T
Takeuchi
Hideo
H
D002413:Cations, Divalent / Q000737:chemistry; D002942:Circular Dichroism; D003300:Copper / Q000737:chemistry*; D008958:Models, Molecular; D008968:Molecular Conformation; D013053:Spectrophotometry; D014364:Tryptophan / Q000737:chemistry
D002413:Cations, Divalent; D003300:Copper; D014364:Tryptophan